Many household appliances are equipped with mechanical timers to control their operation. Examples include dishwashers, icemakers, clotheswashers and dryers, wall and outlet timers, microwave ovens, and various other appliances.
While there is thus a diverse variety of applications for timers, most timers have a similar general structure. Typically, the timer includes a wheel or drum outfitted with cam surfaces. Spring metal switch arms are mounted to ride on these cam surfaces to be raised and lowered from the wheel or drum surface in response to the elevation of the cam surfaces.
A timing motor is typically coupled to rotate the cam wheel or drum, such that the switch arms are raised or lowered in accordance with a predefined regular pattern that is defined by the elevation of the cam surfaces on the wheel or drum. In some timers, the timing motor moves the wheel or drum by causing drive pawls to oscillate and move the cam wheel or drum forward in a step-by-step fashion. In other timers, the timing motor is connected through a gear train to a toothed surface on the cam wheel or drum to rotate the cam wheel or drum in a continuous manner. In either case, the timing motor and its stator, rotor and windings is typically a separately assembled part, housed in a separate housing from the drive assembly; as a consequence, the combination of the timing motor and gear train are fairly substantial in size, and form a large part of the volume and weight of the timer.
The switch arms inside the timer are typically mounted in pairs such that cam-actuated motion of either or both switch arms of a pair causes the pair of arms to make or break and electrical contact therebetween. The switch arms thus form an electrical switch that controls the operation of the appliance. In some timers, switch arms are mounted in groups of three so as to form a single pole, double throw switch or other more complex switching arrangement.
The contacting surfaces of the arms are often coated with expensive metals such as silver alloy to facilitate good contact between the arms and minimize the effects of corrosion. To further facilitate contact between the arms, in some timers a contact rivet is included on each arm, extending toward the opposite arm, such that contact is made between the rivets on the switch arms. To avoid the cost of making and assembling this additional contact rivet, in other timers the arms are stamped with a "dimple", i.e., a raised section of metal that extends toward the opposite arm to form a contact surface. This approach is useful in containing costs where it can be applied; however, where the switch arms are mounted in a group of three, the central switch arm cannot be dimpled to form a contact, since the dimple can only extend in one direction relative to the surface of the central switch arm and the central switch arm must make contact with the arms above and below it. Accordingly, when three switch arms are stacked in this manner, the central switch arm must be outfitted with a contact rivet in order to have surfaces that extend toward both neighboring arms, increasing costs.
In a typical timer there are multiple switches and thus multiple groups of two or more switch arms that interact with the cam surfaces on the cam wheel or drum. In such timers, often the switch arms are mounted in "wafers"; that is, the respective upper arms of each switch is mounted in a first wafer, and the respective lower switch arms of each switch is mounted in a second wafer. The wafers are typically formed of plastic molded over the ends of the switch arms opposite their cam-actuated surfaces. To mount the switch arms for actuation by the cams of the wheel or drum, the wafers are stacked atop each other, and affixed to the timer housing, so that the arms are suspended in a specific position relative to the wheel or drum of the timer.
To assure proper switch functions, the position of the switch arms relative to the wheel or drum, must be controlled to fairly tight tolerances. This means that the size of the wafers, and the position of the switch arms in the wafers, and the mountings to which the switch wafers are mounted, must also be controlled to tight tolerances. Unfortunately, where two or three wafers are stacked to create switch groups of two or three arms, the necessary tolerances become difficult to satisfy, most particularly because it is difficult to maintain a tight tolerance in the switch mounting surfaces that span a long distance, e.g., the entire height of a stack of three wafers. Manufacturing wafers and mountings to sufficiently tight tolerances is thus difficult and expensive.
The switch arms in a wafer are typically made of the same material. Inexpensive metals such as alloy brass are typically used to make switch arms for low current applications. In higher current applications, more expensive, more highly conductive metals such as copper alloy are used to minimize resistance and the resultant heat and energy loss. Unfortunately, even if only one pair of switch arms carries high current, the need for more expensive metals in the switch arms substantially increases the cost of the timer.
The appliance operator typically sets the timer using a knob that extends outside of the timer housing and can be grasped by the operator. In a typical clotheswasher timer, for example, the operator rotates the knob in a forward direction, thereby rotating the cam wheel or drum in a forward direction, until the cam wheel or drum is an appropriate initial position to begin a timed operation cycle. The user then presses a button, or moves the knob axially to initiate the cycle and also start the timing motor.
As is familiar to most users of household appliances, a substantial clatter is generated by the interaction of the cam-operated switches and drive pawls and/or any one-way or ratchet clutch when the timer is advanced to the appropriate position to begin a cycle. For example, the drive pawls click across the pawl-driven surfaces of the cam wheel or drum as the wheel or drum is advanced, and at the same time, the cam operated switch arms click as they are opened and closed by the cam surfaces as the wheel or drum is rotated, and any one-way clutch also clicks. The resulting noise is unpleasant, and is accompanied by substantial irregular tactile feedback.
A second difficulty is that the timer must be set by rotation in a single direction. This constraint arises from the fact that the cam surfaces on the drum or wheel typically are formed with sharp drop-offs so that switches are closed or opened rapidly. Reverse rotation of the cam will cause the cam surfaces on the drum or wheel to bind against the switch arms, preventing further reverse rotation and potentially damaging the timer. To prevent damage by reverse rotation timers often include a rachet pawl or other mechanism to block reverse rotation; of course, this structure only enhances the clatter generated during forward rotation of the timer for setting.
Recently, so-called "quiet set" drum-type timers have been introduced. In these timers, a mechanism lifts the switch arms and drive pawls from the surface the drum to disengage the drum from the pawls during setting. This permits the drum to be rotated manually without clatter from the pawls and switch arms, and also permits bi-directional rotation during setting because the pawls and arms are disengaged from the drum surface.
Unfortunately, users have become accustomed to receiving tactile feedback when setting a timer, and may prefer to receive such feedback. A "quiet set" timer, therefore, may be perceived as undesirable as compared to a timer that does provide tactile and audible feedback such as a prior non-"quiet set" timer.